Transparent films are being increasingly used as coatings for various glass and plastic articles, for instance, optical lenses, goggles, face shields, face plates for helmets, automobile windshields, and the like. Organic polymers, such as polyurethanes, are particularly useful in providing self-supporting surfaces or coatings for such applications, in that these transparent materials can provide high strength and improved abrasion resistance to the surface. Such organic polymers, however, have a high susceptibility to fogging on their surfaces, due to formation of droplets of water condensation, which simple wiping cannot adequately ameliorate. Additionally, in many applications, and particularly those contemplated herein, wiping the water droplets from the surface is not possible or practicable.
In order to prevent this fogging, it is known to use various surface active agents to provide anti-fog properties to such articles. For example, hydrophilic agents have been added to polyurethanes in order to impart anti-fog properties. U.S. Pat. Nos. 4,551,484 and 4,609,688 to Radisch et al., disclose anti-fog coating compositions for transparent surfaces which include a three-dimensional cross-linked polyurethane having a free surface active agent disposed within open domains in its cross-linked structure. The coating compositions are prepared by reacting isocyanates with polyfunctional polyols to obtain a polyurethane, and subsequently contacting the thus prepared polyurethane with a hydrophilic surface-active agent in order to diffuse molecules of the surface-active agent into the interior of the coating.
The surface-active agent, however, is not chemically reacted into the polyurethane, but is instead physically disposed within the polymeric structure. As such, the cured coating is susceptible to undesirable leaching and erosion of the surfactant, thereby decreasing the anti-fog properties of the coating composition.
It has also been proposed to react surface active agents into a polyurethane coating composition in order to impart anti-fog properties to the coating composition. For instance, U.S. Pat. No. 3,822,238 to Blair et al. discloses the addition of sulfonated "resins" to polyurethanes in order to prepare coatings with various properties including anti-fog characteristics. The resins are prepared from diols or diamines reacted with di-carboxylic acid esters, followed by sulfonation of double bonds or quarternization of amines. The resins are intended to increase the hydrophilic character and water absorption of the polyurethane coatings by reacting into the polyurethane backbone in an end-to-end fashion, rather than as pendent groups. Such resins which react in an end-to-end fashion, as opposed to remaining pendant at the end of the polyurethane chain, cannot provide for a clear delineation of hydrophilic and hydrophobic groups and in this respect do not behave as surfactants, i.e., they do not provide cooperation between distinct hydrophilic and hydrophobic portions to reduce interfacial tension.
U.S. Pat. No. 4,745,152 to Fock et al. discloses polyurethane compositions useful as coatings for transparent substrates with improved self-healing properties and prevention against formation of surface moisture. The polyurethane compositions are prepared from a reaction of an isocyanate with a polyol mixture including a difunctional sulfonated polyether polyol and a trifunctional polyol. Such a polyurethane composition incorporates only polyol combinations which impart hydrophilic character to the coating, and does not further incorporate into the composition a surfactant material.
None of the above-mentioned patents suggest or describe that their compositions may provide permanent fog resistance properties, i.e. fog resistant properties which last after repeated washings or extended soaking in water, nor do they teach or suggest that the effectiveness of their coatings can last more than a few hours of use.
Additionally, it is known to incorporate non-ionic surfactants containing reactive functional groups into polyurethanes prepared with polyvinylpyrrolidone as a hydrophilic agent. For example, U.S. Pat. No. 4,467,073 to Creasy discloses anti-fog coating compositions incorporating an isocyanate prepolymer which is reacted with a polyvinylpyrrolidone polymer, the reaction product thereof being subsequently reacted with a non-ionic surfactant having reactive groups for reacting with the isocyanate, for instance, hydroxyl reactive groups. Polyvinylpyrrolidone polymers, however, while serving to increase the hydrophilicity of the polyurethane matrix and improve anti-fog properties, also reduce the scratch-resistance, chemical resistance, water sensitivity, and durability of the cured polyurethane surface. Thus, although these compositions, when cured, have been known to provide anti-fog properties, their solvent sensitivity, flexibility and scratch resistance properties are less than desirable.
Thus, a need exists for a polyurethane composition which when cured provides enhanced chemical resistance and scratch resistance in addition to long lasting, permanent anti-fog properties and which are not easily susceptible to erosion or leaching out of the surfactant. The present invention provides such a composition by incorporating into the polyurethane backbone hydrophilic groups as well as pendent surfactant groups.